Fix ty invalid assignment

[kadubhumika]

Description

Fixes the remaining ty invalid-assignment and related type-checking diagnostics across multiple files.
Fixes: #15204

Changes made

• Fixed type annotations and optional Node handling in linked list implementations.
• Fixed type issues in the segment tree and binomial heap.
• Fixed typing for automatic differentiation and minimum cut implementations.
• Added explicit urllib.request import.
• Fixed the CPU scheduling selection type-checking issue.
• Added necessary assertions for image pixel access.

Validation

ty check --exclude-scripts

Result:

All checks passed!

[algorithms-keeper]

Closing this pull request as invalid

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[cclauss]

@priya-sundaram-dev Your review, please.

[priya-sundaram-dev]

Reviewed post-merge — this is exactly the minimal-surface-area approach #15204 was asking for. 👍

• The assert x is not None narrowing on img.load() / bottom_root / min_node is the right idiom: it tells the type checker what the code already guarantees at that point, without changing runtime behavior for valid inputs.
• cast(T, None) in non_recursive_segment_tree cleanly drops the Any escape hatch while keeping the placeholder-fill pattern intact.

Thanks @kadubhumika for knocking out all 23 spots in one pass. Nicely scoped.

[kadubhumika]

Thanks for the merge and the great feedback @cclauss and @priya-sundaram-dev ! 

I really enjoyed working on this and would love to keep contributing. If there are any other type-checking issues, open bugs, or features you need help with, please let me know, I’d be happy to jump on them!

[cclauss]

Some typing-related issues:

• Tracking: complete the ty gradual-typing baseline (un-ignore rules; PEP 723 blocker astral-sh/ty#691) #15187
• sorts: make algorithms sort any comparable items, not just ints #15234

[kadubhumika]

Thanks for the merge and the kind words! I will jump right into #15187 and #15234 next. @cclauss